Update test_runner.py (#510)

diff --git a/mobly/test_runner.py b/mobly/test_runner.py
index 909385b..c459d32 100644
--- a/mobly/test_runner.py
+++ b/mobly/test_runner.py
@@ -117,7 +117,7 @@
         '--list_tests',
         action='store_true',
         help='Print the names of the tests defined in a script without '
-        'executing them. If the script ')
+        'executing them.')
     parser.add_argument(
         '--tests',
         '--test_case',